Choke coils are used in rectifying/smoothing circuits for smoothing an output of a switching power supply as well as normal mode noise filters. The choke coil cores are subject to a biasing DC magnetic field and an AC magnetic field is applied thereto in an overlapping manner. There. fore, the choke coil cores are required to have a wide unsaturation region of from 0 to about 25 Oe in their B-H hysteresis loop, that is, a flattened out B-H loop with low magnetic permeability. Cores having high permeability do not perform as choke coils since they are saturated with a slight change of applied magnetic field intensity.
In order that smoothing choke coils exhibit stable DC overlapping capability against any load variation and that normal mode choke coils on the primary side exhibit stable properties at power-frequency, choke coil cores are required not to lower their magnetic permeability at high current flow (or high magnetic field) and to maintain isopermeability in that permeability is approximately constant over the range of 0 to about 25 Oe. To reduce the size of choke coils, it is important that magnetic core materials have high saturation magnetic flux density and reduced losses.
Amorphous iron base alloys are promising soft magnetic materials having a high saturation magnetic flux density suitable as choke coil magnetic core materials. For example, Japanese Patent Application Kokai (JP-A) No. 52557/1985 discloses a low.loss amorphous magnetic alloy of a Fe-Si-B alloy composition having Cu added thereto. The amorphous magnetic alloy is heat treated at a temperature below the crystallization temperature for reducing core losses. However, such heat treatment is not successful in achieving low permeability and the resulting amorphous alloy has a so narrow unsaturation region that it might be saturated even at 20 Oe, suggesting that the alloy is not useful as cores. Due to its high magnetostriction, the alloy can give rise to a beat problem when formed into cores.
JP-A 39347/1989 discloses an iron base soft magnetic alloy which is prepared by heat treating an amorphous alloy for creating fine crystal grains. It is described that better magnetic properties are obtained with a grain size of up to 50 nm, most often with a mean grain size of 2 to 20 nm. The iron base soft magnetic alloy disclosed in this publication as having fine crystal grains, however, is not suitable as choke coil core material since it has too high permeability and is so narrow in unsaturation region that it can be saturated even at 20 Oe. This iron base soft magnetic alloy contains Cu and Nb or the like as essential elements in a total content as high as about 4 atom %, at which it is difficult to prepare ribbon shaped amorphous alloy.
In order to reduce the permeability of magnetic cores formed from such high permeability soft magnetic alloys, it was generally attempted to form cut cores or to form a gap in a core, thereby forming a gap radially traversing the magnetic path for flattening the B-H loop. For example, a wound core obtained by winding a soft magnetic thin strip is provided with a gap by impregnating the wound core with resin, radially cutting the core to form core segments, and mating the core segments together to form a core.
However, when the wound core is cut, the thin strip can be deformed at the cutting section so that the thin strip turns come in contact where heat generates during operation, resulting in increased losses. Further, the resin impregnation introduces stresses into the wound core, resulting in deteriorated magnetic properties and increased core losses. The additional gap forming step reduces manufacture efficiency. Magnetostriction allows generation of beat which can be amplified at the gap.
One typical method for preparing gapless low magnetic permeability cores is by partially crystallizing an amorphous alloy as disclosed in JP-A 169209/1982 and 4016/1988. The alloy of JP-A 24016/1988, however, has poor magnetic properties and increased core losses because it is crystallized only in proximity to its surface and internal stresses are induced within the alloy. The alloy compositions described in these published applications are not successful in reducing magnetostriction, so that magnetic cores formed therefrom suffer from a beat problem.
It was also proposed to reduce the permeability of alloys by forming an oxide layer on their surface. Stresses are induced within the alloys in this case too, leading to increased coercivity and eventually poor magnetic properties and increased core losses. Although low magnetic permeability is achieved by these oxide coated alloys, the alloys under a high magnetic field (or high electric current) applied have a magnetic permeability which is substantially lower than the permeability at the origin of the B-H loop, indicating that the alloys do not possess iso. permeability.
Moreover, iron base amorphous soft magnetic alloys as mentioned above have a problem in a practical frequency band of 100 kHz to 1 MHz where minor loops are drawn in an overlapping manner that effective permeability is subject to resonance due to magnetostriction when a DC magnetic field is overlappingly applied, failing to stabilize effective permeability.